ToNumberField

ToNumberField[a,θ] expresses the algebraic number a in the number field generated by θ. 

ToNumberField[{a1,a2,…},θ] expresses the ai in the field generated by θ.

ToNumberField[{a1,a2,…}] expresses the ai in a common extension field generated by a single algebraic number.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.